It is known to evaluate and to monitor the quantity of ice on a surface of an aircraft, such as a wing, for example.
An optical system is used for this comprising a still camera and a laser. The beam from the laser passes through the ice and illuminates the surface of the aircraft at a central point. The light rays of the beam are then reflected in all directions from the central point and some of these reflected rays are refracted in contact with the ice/air interface with the result that they are again directed toward the surface of the aircraft so as then to form a refraction spot on the surface of the aircraft at a non-zero distance from the central point. The system then measures the distance between the refraction spot and the central point and obtains the depth of the ice by deducing it from the refraction angle of the ice.
However, for this method, based on the refraction of the rays at the air/ice interface and therefore on the angle of refraction of the ice, to be reliable, it is necessary for the ice surface to be plane. Indeed, any deformation of the air/ice interface leads to unpredictable refraction of the rays and therefore interferes with the measurement of the thickness of the ice. The surface of the ice being liable to assume various shapes on the surface of an aircraft, this method does not guarantee that it will be possible to measure the thickness of the ice on aircraft surfaces in all circumstances.